Generally, it is well known that a richer air-fuel mixture is supplied by a carburetor when automobiles run at altitudes than that obtained during operation at sea level.
It is also known that the concentration or air-fuel ratio of the air-fuel mixture is determined by amounts of air and fuel supplied to the engine. The fuel content of the air-fuel mixture is adjusted by means of an amount of fuel flowing through a fuel jet and an amount of air supplied from an air bleed. It is therefore possible to finally determine an air-fuel ratio of the mixture by controlling the flow rate of air flowing through the air bleed. It is naturally understood that an undesirable high concentration tendency at altitudes is effectively avoided by increasing the flow rate of bleed air in accordance with the increase of the altitude.
As a practical measure for such a control of bleed air, it has been proposed to drive by a bellows sensitive to the atmospheric pressure an air metering valve which in turn meters the air supplied to the main well of the carburetor thereby to make the mixture lean. This system is described in detail in the specification of the U.S. Pat. No. 4,129,622.
When an automobile provided with a carburetor incorporating this type of altitude compensating device runs at altitudes, the air-fuel mixture is inconveniently made lean as the flow rate of the intake air is increased, so that the engine fails to provide the required output power, resulting in a deteriorated driving performance.
This undesirable leaning of the air-fuel mixture is considered to be attributable to the following reason. Namely, as the flow rate of the intake air is increased, the venturi vacuum generated at the venturi of the carburetor is increased to cause a large differential pressure between the venturi vacuum and the atmospheric pressure at the upstream side of the air metering valve constituting an altitude compensating device, so that an excessively large amount of air is allowed to flow into the main well of the carburetor.
In order to avoid this undesirable phenomenon, it has been attempted to design and adjust an altitude compensating device of which altitude compensating characteristics match the state of large flow rate of intake air. This, however, results in an excessively rich mixture when the flow rate of intake air is comparatively small.